260 Mr. R. Hunt on tlie Use o/Hydriodic Salts 



a dilute solution of sulphocyanide of potassium, the sulpho- 

 cyanogen is precipitated of a light yellow colour, and does 

 not subside readily. I have ascertained, by experiment, that 

 this light yellow substance is not sulphocyanogen, but hydro- 

 thiocyanic acid. It is, however, more difficult to convert sul- 

 phocyanogen into this acid by chlorine, than by nitric acid ; 

 chlorine consequently gives the purer substance. 



In conclusion, I would state, that these experiments have 

 been performed in Prof. Graham's laboratory, to whom and 

 to his late assistant Mr. Fownes, I am happy in acknowledging 

 myself indebted for their suggestions during this research. 



University College, June 25, 1840. 



XXXVIII. On the Use ofHydriodic Salts as Photographic 

 Agents. By Mr. ROBERT HUNT. 



[Continued from p. 211, and concluded.] 

 38. On the darkening of the Photograph. 



TV/TR. TALBOT first directed attention, at the last meeting 

 *--* of the British Association, to a peculiarity possessed by 

 some of these kinds of photographs, namely, that they were 

 neither fixed nor otherwise ; but that on exposure to sunshine 

 they changed in their dark parts from a red to a black, the 

 lights of the picture being unaffected by the light. 



39. This singular effect I have proved to be entirely de- 

 pendent on the influence exerted by the less refrangible rays 

 of the solar spectrum in exalting the oxidation of the silver; 

 but a brief statement of some effects produced by the dis- 

 severed rays, will place the matter in a much clearer light. 



40. By allowing a very intense prismatic spectrum, formed 

 by a flint-glass prism, to fall upon any of these photographs 

 which blacken by white light, it will be found that the dark- 

 ening process commences in the red ray, at which point it 

 goes on with the greatest intensity, and is gradually shaded 

 off to the lowest edge of the extreme red ; the shading is also 

 continued through the orange and yellow rays being sharply 

 cut off at that line of the spectrum where the pure green is 

 visible. 



41. As it was not possible to pursue my inquiry on the 

 effects of the spectrum with any degree of satisfaction without 

 a heliostat, an instrument I have not the means of procuring, 



in alkali, the sulphocyanogen is first dissolved, leaving the acid. This ex- 

 plains why sulphocyanogen prepared by nitric acid becomes yellow when 

 treated with alkali, which is not the case with that prepared by chlorine. 



